Query Rewrite With A Nested Materialized View

1. A computer-implemented method, comprising steps of: selecting, from among a hierarchy of materialized views, only first-tier materialized views that depend directly on a base table; establishing a set of candidate materialized views that consists only of the first-tier materialized views; based on a cardinality of a nested materialized view that is a descendant of a particular first-tier materialized view, selecting the particular first-tier materialized view from among the set of candidate materialized views even though the particular first-tier materialized view does not have a lowest cardinality among cardinalities of first-tier materialized views in said set of candidate materialized views; based on said selecting, rewriting an existing query statement, which references said base table, into a rewritten query statement that references said particular first-tier materialized view instead of said base table; wherein said first-tier materialized view is a parent materialized view to another materialized view; and wherein said method is performed by one or more computing devices.

2. The method of claim 1 , wherein said nested materialized view is nested at a final nesting level of one or more nesting levels associated with said parent materialized view.

3. The method of claim 1 , wherein said selecting includes comparing cardinalities of certain nested materialized views associated with any of said set of candidate materialized views.

4. The method of claim 3 , wherein comparing cardinalities includes comparing cardinalities of a set of certain nested materialized views at a final nesting level.

5. The method as recited in claim 1 , further comprising a step of persistently storing information relating to relationships between each candidate materialized view of said set of candidate materialized views and respective one or more nested materialized views of said each candidate materialized view, and cardinality values and nesting levels associated with said each candidate materialized view and respective one or more nested materialized views.

6. A computer-implemented method, comprising: selecting a certain candidate materialized view from among a plurality of candidate materialized views, wherein each candidate materialized view of said plurality of candidate materialized views is associated with a respective nested materialized view at a final nesting level; wherein said selecting includes: (a) making a comparison of a cardinality of each respective nested materialized view at the final nesting level, and (b) selecting the candidate materialized view with a lowest cardinality based on said comparison; and based on said selecting, rewriting an existing query statement, which references a base table upon which a first-tier ancestor of the certain candidate materialized view directly depends, with a rewritten query statement that references the first-tier ancestor instead of the base table; and wherein the method is performed by one or more computing devices.

7. A non-transitory computer-readable storage medium storing one or more sequences of instructions, wherein execution of the one or more sequences of instructions by one or more processors causes the one or more processors to perform steps of: selecting, from among a hierarchy of materialized views, only first-tier materialized views that depend directly on a base table; establishing a set of candidate materialized views that consists only of the first-tier materialized views; based on a cardinality of a nested materialized view that is a descendant of a particular first-tier materialized view, selecting the particular first-tier materialized view from among the set of candidate materialized views even though the particular first-tier materialized view does not have a lowest cardinality among cardinalities of first-tier materialized views in said set of candidate materialized views; based on said selecting, rewriting an existing query statement, which references said base table, into a rewritten query statement that references said particular first-tier materialized view instead of said base table; and wherein said first-tier materialized view is a parent materialized view to another materialized view.

8. The non-transitory computer-readable medium of claim 7 , wherein said nested materialized view is nested at a final nesting level of one or more nesting levels associated with said parent materialized view.

9. The non-transitory computer-readable medium of claim 7 , wherein said selecting includes comparing cardinalities of certain nested materialized views associated with any of said set of candidate materialized views.

10. The non-transitory computer-readable medium of claim 9 , wherein comparing cardinalities includes comparing cardinalities of a set of certain nested materialized views at a final nesting level.

11. The non-transitory computer-readable medium as recited in claim 7 , wherein the steps further comprise persistently storing information relating to relationships between each candidate materialized view of said set of candidate materialized views and respective one or more nested materialized views of said each candidate materialized view, and cardinality values and nesting levels associated with said each candidate materialized view and respective one or more nested materialized views.

12. A non-transitory computer-readable storage medium storing one or more sequences of instructions, wherein execution of the one or more sequences of instructions by one or more processors causes the one or more processors to perform steps of: selecting a certain candidate materialized view from among a plurality of candidate materialized views, wherein each candidate materialized view of said plurality of candidate materialized views is associated with a repective nested materialized view at a final nesting level; wherein said selecting includes: (a) making a comparison of a cardinality of each respective nested materialized view at the final nesting level, and (b) selecting the candidate materialized view with a lowest cardinality based on said comparison; and based on said selecting, rewriting an existing query statement, which references a base table upon which a first-tier ancestor of the certain candidate materialized view directly depends, with a rewritten query statement that references the first-tier ancestor instead of the base table.